The present invention is generally directed to the field of fuel cells and more particularly to solid oxide reversible fuel cell systems.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. Electrolyzer cells are electrochemical devices which can use electrical energy to reduce a given material, such as water, to generate a fuel, such as hydrogen. The fuel and electrolyzer cells may comprise reversible cells which operate in both fuel cell and electrolysis mode.
In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit. A solid oxide reversible fuel cell (SORFC) system generates electrical energy and reactant product (i.e., oxidized fuel) from fuel and oxidizer when operating in a fuel cell or discharge mode and generates the fuel and oxidant using electrical energy when operating in an electrolysis or charge mode.
When an SORFC system is operating in the electrolysis mode, a mixture of water and hydrogen is generated and may then be circulated through the fuel generation chambers of the cells within the stack. The mixture of water and hydrogen must be separated prior to delivery of the hydrogen to storage and/or to the customer. Conventionally, a gravity separator is used to separate the hydrogen from the mixture. In that case, the mixture must be cooled to condense the water in the mixture prior to separation and must then be re-evaporated prior to reintroduction to the SORFC stack. The separated hydrogen is typically humid following gravity separation and must be dried prior to storage and/or use as a fuel. Additionally, a compressor is typically used to pressurize the dried hydrogen prior to storage and/or use as a fuel. Each aspect of the process of condensing, re-evaporating, drying, and compressing requires the use of additional system components and the expenditure of additional energy in the SORFC system, thereby adding bulk to and decreasing the energy efficiency of the system.